1. Field of the Invention
This invention relates generally to borehole flow meters and particularly to borehole flow meters for measuring both the rate and direction of fluid flow.
2. Description of the Prior Art
Borehole flow meters have been used in the oil and gas industry for many years. One example of prior art flow meters is described in U.S. Pat. No. 3,630,078 issued in the name of Jean-Loup Bonnet. The flow meter described in that patent has a spinner blade or screw attached to a shaft mounted in a body member. Borehole fluid turns the spinner and the shaft to which it is attached. Permanent magnets disposed on the shaft actuate a reed relay disposed in the body adjacent the magnets. The turning on and off of the reed relay in response to the turning of the permanent magnets on the shaft serves to provide an indication of the flow rate of the fluid driving the spinner blade and consequently gives an indication of the fluid flow rate in the borehole.
While the borehole flow meter described in U.S. Pat. No. 3,630,078 has provided good measurements over the years for measuring borehole fluid flow rates, it has been found that providing permanent magnets on the shaft driven by the spinner is disadvantageous in at least two respects.
First, the permanent magnets attract magnetic particles which may be found in the borehole fluid. This is a problem regardless of the efficacy of sealing the shaft from borehole fluid. Such borehole fluid often is at high pressure and minute particles from the fluid may find their way through bearing and/or pressure sealing means to the space between the shaft and the body. Such particles may be attracted to the permanent magnets on the shaft and hinder the free and easy turning of the shaft.
A second problem has to do with the weight of the shaft caused by placing the permanent magnets on the shaft. Such weight adds to the inertia of the spinner/shaft system which negatively affects the responsiveness of the apparatus to changes in flow rates.
Another problem characteristic of all known borehole flow meters relates to their inherent inability to detect fluid flow rates in both directions. Borehole fluid may of course travel from the bottom of the hole upwardly for the case of a producing well where fluids are produced from a perforated zone and are flowing upwardly in the well. On the other hand, the fluid may be flowing from the top part of the well to the bottom part of the well for the case of an injection well where fluid is pumped from the top part of for injection via perforations in the bottom part of the hole.
Still another case where fluid flow may be in both directions is in a deviated borehole, so often found in offshore boreholes. In certain deviated boreholes even though the flow may be primarily in one direction from the bottom to the top of the hole, in certain locations in the hole, say in a lateral portion of it, the flow may be in both directions.
3. Identification of the Objects of the Invention
It is a primary object of the invention to provide a borehole apparatus for measuring the direction of the flow of borehole fluid.
It is another object of the invention to provide borehole fluid flow measuring apparatus which measures not only the direction of fluid flow, but also its speed or rate in either direction.
It is a still further object of the invention to provide borehole fluid flow measuring apparatus which because of its construction, is capable of measuring fluid flow rates at extremely low levels.
It is another object of the invention to provide an extremely light spinner shaft which has extremely low inertia and friction opposing rotation so as to enable it to measure extremely low flow rates.